To reduce the number of parts and to easily secure an arranging space by fixing a reverse bucket to the position for covering an opening part of a nozzle deflector and also to the upper position for releasing it.

A reverse bucket 50 is supported on a propulsion duct 12 so as to be vertically oscillated, a lock mechanism 70 is provided on the reverse bucket 50, a jump preventing stopper and a bucket falling preventing stopper 80 are provided on the propulsion duct 12, the lock mechanism 70 is engaged with the jump preventing stopper in the astern position to regulate jump of the reverse bucket 50, while, the lock mechanism is engaged with the bucket falling preventing stopper 80 in the forward position to regulate slip of the reverse bucket 50.